Stray field equalization for improved domain expansion reading

ABSTRACT

The present invention relates to magneto-optical recording technique by which an improved domain expansion reading is achieved. A mark region is recorded as a sub-mark portion and an adjacent sub-space portion, wherein the sum of said predetermined first and second lengths is changed in dependence said pattern of marks and spaces. The proposed write strategy allows to write a long run length with sub-mark and/or sub-space lengths selected independent of the channel bit length, such that a long run length can be written with few well-chosen domains with a stray field larger than the minimum field for MAMMOS readout. In this way, differences in readout conditions for all combinations of short and long run lengths can be eliminated, resulting in substantially improved power margins for random data.

The present invention relates to a method, apparatus and recordingmedium for recording an information as a pattern of marks and spaces ona recording track. In particular, the present invention relates to arecording technique for a domain expansion system, such as a MagneticAMplifying Magneto-Optical System (MAMMOS).

In magneto-optical storage systems, the minimum width of the recordedmarks is determined by the diffraction limit, i.e. by the NumericalAperture (NA) of the focussing lens and the laser wavelength. Areduction of the width is generally based on shorter wavelength lasersand higher NA focussing optics. During magneto-optical recording, theminimum bit length can be reduced to below the optical diffraction limitby using Laser Pulsed Magnetic Field Modulation (LP-MFM). In LP-MFM, thebit transitions are determined by the switching of the field and thetemperature gradient induced by the switching of the laser. For read-outof the small crescent shaped marks recorded in this way, Magnetic SuperResolution (MSR) or Domain Expansion (DomEx) methods have to be used.These technologies are based on recording media with severalmagneto-static or exchange-coupled RE-TM layers. According to MSR, aread-out layer on a magneto-optical disk is arranged to copy a mark fromthe storage layer only in a small region of the readout spot and maskadjacent bits during reading, while, according to domain expansion, adomain in the center of a spot is expanded. The advantage of the domainexpansion technique over MSR results in that bits with a length belowthe diffraction limit can be detected with a similar signal-to-noiseratio (SNR) as bits with a size comparable to the diffraction limitedspot. MAMMOS is a domain expansion method based on magneto-staticallycoupled storage and read-out layers, wherein a magnetic field modulationmay be used for expansion and collapse of expanded domains in theread-out layer.

However, when long run lengths are written in a MAMMOS medium, themagnetic stray-field in the center of the domain corresponding to thelong run length is weaker than at the boarders thereof (in thetangential direction). At a particular “critical length”, the magneticstray-field in the center of the run length becomes insufficientlystrong to generate a MAMMOS signal in that area, i.e. to obtain a copieddomain in the read-out layer. This results in an erroneous bit stream.The problem can be solved by increasing the reading power of the laser,thus increasing the total temperature and thereby the local magneticstray-field of the storage layer and at the same time decreasing thecoercivity of the readout layer. If the increase in the magneticstray-field and the decrease in coercivity are sufficient, thepreviously missing MAMMOS signal will be generated. However, thisprocedure increases the thermal copy window which determines theresolution for read-out, such that extra false MAMMOS signals may begenerated due to overlapping effects.

Document JP-A-2000-260079 suggests a MAMMOS recording system in which abinary information of one bit is allotted to a magnetic section patternconstituted by a combination of two magnetic sections havingmagnetizations with opposite directions, such that a recordinginformation that continues for two or more bits is formed in therecording layer as a series of magnetic section patterns with oppositemagnetization. Thereby, a homogeneous stray-field is obtained,irrespective of the position of a respective read-out domain, even if itis located in the center of a continuous recording information. Hence,each unit of recording information can be reliably transferred to theplayback layer. In particular, a mark region is recorded as a sequenceof a sub-mark region having a length L1 and a following short sub-spaceregion having a length L2. The ratios L2/L1 between the length of thesub-space region and the length of the sub-mark region is suggested tobe in the range of 0.1 to 0.9.

For MAMMOS readout, the sum of the external field and the stray fieldfrom the bit pattern in the storage layer should be larger than thecoercive field of the readout layer:H _(ext) +H _(stray,storage) >H _(c,readout)  (1)Because the stray field increases and the coercive field decreases withincreasing temperature (proportional to laser power), a minimumtemperature T_(min) (or laser power) is required to fulfill thiscondition. On the other hand, if the laser power becomes too large, thedimensions of the area where the temperature is higher than this T_(min)are so large that overlap with neighboring bits will occur. This willlead to false, additional peaks, such that a wrong number of peaks willbe detected during readout of long mark run lengths, Moreover, smallspaces can not be detected at all. Therefore, the laser power should becontrolled in such a way that the temperature in the center of the spotis just above T_(min). The stray field also depends on the length of thewritten domain (and its surroundings).

FIG. 2A shows a stray field (in the normal direction z with respect tothe recording surface) of a written domain or mark region along thetrack direction for different domain lengths or bit lengths ranging from30 to 1000 nm. Furthermore, FIG. 2B shows a diagram indicating themaximum stray field H_(z,max) and the stray field H_(z,center) at thecenter of the domain as a function of the bit length. As can be seenfrom FIGS. 2A and 2B, the stray field decreases for bit lengths largerthan 100 nm, especially near the center of the domain. This means thatwhen the readout conditions are optimized e.g. for a channel bit lengthb=100 nm, larger domains will not show any MAMMOS signals. When using ahigher read power or larger external field, only the MAMMOS peaks fromthe center of the large domain will be missing. However, small domainscan no longer be resolved.

FIG. 3 shows a diagram indicating the stray field versus the trackdirection for a channel bit length b=100 nm for a sequence of domains ofa length corresponding to one channel bit length, i.e. I1 carriers(dashed line), and a continuous domain of a length corresponding to fivechannel bit lengths, i.e. an I5 carrier (solid line). In the upper partof FIG. 3, magnetization patterns corresponding to the stray fieldwaveforms are indicated, wherein four subsequent plus signs indicated amark region of a length corresponding to one channel bit length andwherein four minus signs indicated a space region of a lengthcorresponding to one channel bit length. Curves 1 and 2 representcoercive field profiles at different values of H_(c,readout)−H_(ext). InFIG. 3, the first situation corresponds to H_(c,readout)−H_(ext) justbelow 27 kA/m (curve 1). In this case only the tip of the thermalprofile is used. When H_(c,readout)−H_(ext) is around 12 kA/m in thehottest part of the spot (curve 2), the second situation applies. Here,I1 spaces (i.e. spaces with a length corresponding to one channel bitlength) cannot be observed because the coercive field profile alwaysoverlaps with the stray field of neighboring marks. Stated moregenerally, for MAMMOS readout with the best resolution/power margin thedifference between the largest value of the stray field and the lowestvalue among all combinations of run lengths should be as small aspossible.

It is therefore an object of the present invention to provide a method,apparatus and medium for recording on a magneto-optical medium, by meansof which the MAMMOS reading resolution and power margin can be improved.

This object is achieved according to the invention by providing arecording method as claimed in claim 1, a recording apparatus as claimedin claim 9, and a recording medium as claimed in claim 13.

Accordingly, equalization of stray fields from (combinations of) longand short run lengths can be achieved by suitably setting the lengths ofthe sub-mark and/or sub-space portions independent of the actual channelbit length. A long run length can be written with a few separate‘up’-domains with length and separation adjusted to the coercivityprofile in order to give overlap at the required times. There is no needto use a periodic bit pattern inside a run length, as suggested in theknown writing strategies. The right combinations can eliminatedifferences in readout conditions for all combinations of short and longrun lengths, resulting in much improved margins for random data.

Increased flexibility is gained by using larger L2/L1 ratios (smallsub-marks for smallest mark run lengths). The surrounding or neighboringrun lengths influence the stray field of a domain structure due to thegenerated magnetic dipole fields with long range.

Therefore, the write strategy for a run length should in principle beadapted based on the previous and the following data The onlyrequirement for MAMMOS readout is that there should be an overlapbetween the coercive field and the stray field from the bit patternwhere MAMMOS expansion should occur.

As a further advantage of the proposed solution, the effective length ofa run length can be reduced at its end(s). In this way, the resolutionand/or power margin can be further improved, because an unwanted overlapof the stray field with the coercivity profile is suppressed.

It is to be noted that all other laser power and field control methodsbased on (indirect) measurements of the overlap (or the copy window) canstill be used. The sub-mark portions and sub-space portions of long runlengths can be written in such a way that the overlap around e.g. thecenter of the run length is sensitive to a certain range of laser powersor external fields. The locally reduced power margin at the center mightotherwise lead to a missing peak if the laser power is slightly too low,or vice versa. When well chosen, the proposed write strategy can be usedto complement or refine other control methods. Specifically, the sum ofsaid predetermined first and second lengths may be set to be greaterthan a channel bit length. Then, the number of the sub-mark portions inthe concerned mark region can be made smaller than the number of channelbits which correspond to the run length of the mark region. As anexample, a mark region with a run length corresponding to five channelbits may be written with three sub-mark portions separated bycorresponding sub-space portions.

As a further improvement, the distance between the storage layer and thereadout layer may be set based on a difference between the largest andthe lowest values of a stray field along said mark region, to therebyset the stray field level at the readout layer.

Other advantageous developments are defined in the dependent claims.

In the following a preferred embodiment of the present invention will bedescribed in greater detail with reference to the accompanying drawingfigures, in which:

FIG. 1 shows a schematic block diagram of a MAMMOS disk player accordingto the preferred embodiment,

FIG. 2A shows a diagram of a stray-field component perpendicular to thedisk versus a track direction for different bit or run lengths;

FIG. 2B shows a diagram of a stray-field component versus bit orrun-length of a mark region;

FIG. 3 shows a diagram indicating stray field characteristics of I1carriers and an I5 carrier, and different coercive field profiles;

FIG. 4 shows a stray field characteristic and overlap for a specificwrite pattern according to a first example of the write scheme accordingto the preferred embodiment;

FIG. 5 shows a stray field characteristic and overlap for a specificwrite pattern according to a second example of the write schemeaccording to the preferred embodiment;

FIG. 6A shows a stray field characteristic and overlap for a specificdomain pattern according to a third example of the write schemeaccording to the preferred embodiment; and

FIG. 6B shows a stray field characteristic and overlap for a specificdomain pattern according to the third example of the write schemeaccording to the preferred embodiment, at a lower coercive profile.

The preferred embodiment will now be described on the basis of a MAMMOSdisk player as indicated in FIG. 1. FIG. 1 schematically shows theconstruction of the disk player. The disk player comprises an opticalpick-up unit 30 having a laser light radiating section for irradiationof a magneto-optical recording medium 10, such as a magneto-opticaldisk, with light that has been converted, during recording, to pulseswith a period synchronized with code data and a magnetic field applyingsection comprising a magnetic head 12 which applies a magnetic field ina controlled manner at the time of recording and if required also duringplayback on the magneto-optical recording medium 10. In the opticalpick-up unit 30 a laser is connected to a laser driving circuit whichreceives recording pulses from a recording pulse adjusting unit 32 tothereby control the pulse amplitude and timing of the laser of theoptical pick-up unit 30. The recording pulse adjusting circuit 32receives a clock signal from a clock generator 26 which may comprise aPLL (Phase Locked Loop) circuit.

It is noted that playback may not require a magnetic field if zero fieldMAMMOS is used. Furthermore, it is noted that, for reasons ofsimplicity, the magnetic head 12 and the optical pickup unit 30 areshown on opposite sides of the disk 10 in FIG. 1. However, according tothe preferred embodiment, they should be arranged on the same side ofthe disk 10.

The magnetic head 12 is connected to a head driver unit 14 and receives,at the time of recording, code-converted data via a timing adjustingcircuit, such as a phase adjusting circuit 18, and a sub-mark adjustingcircuit 22 from a modulator 24. The modulator 24 converts inputrecording data to a prescribed code, and the sub-mark adjusting circuit22 converts each mark region of the prescribed code into a sub-markportion or region and a sub-space region, while a first predeterminedlength L1 of the sub-mark portion and a second predetermined length L2of the sub-space portion are set based on the pattern of marks andspaces to be written to the disk 10. In particular, the sub-markadjusting circuit 22 may be adapted to set the first and secondpredetermined lengths L1 and L2 in such a manner as to achieve ormaintain a predetermined stray field characteristic required for anadequate detection of the mark and space pattern to be written. It isnoted that the sub-mark adjusting circuit 22 may as well be arrangedwithin the modulator 24.

At the time of playback, the head driver 14 receives a clock signal viaa playback adjusting circuit 20 from the clock generator 26, wherein theplayback adjusting circuit 20 generates a synchronization signal foradjusting the timing and pulse amplitude applied to the magnetic head12. A recording/playback switch 16 is provided for switching orselecting the respective signal to be supplied to the head driver 14 atthe time of recording and at the time of playback. However, thisswitching feature is not required for zero field MAMMOS, since then nomagnetic field has to be applied during playback.

Furthermore, the optical pick-up unit 30 comprises a detector fordetecting laser light reflected from the magneto-optical recordingmedium 10 and for generating a corresponding reading signal applied to adecoder 28 which is arranged to decode the reading signal to generateoutput data. Furthermore, the reading signal generated by the opticalpick-up unit 30 is supplied to a clock generator 26 in which a clocksignal obtained from embossed clock marks of the magneto-opticalrecording medium 10 is extracted, and which supplies the clock signalfor synchronization purposes to the recording pulse adjusting circuit32, the playback adjusting circuit 20, the sub-mark adjusting circuit 22and the modulator 24. In particular, a data channel clock may begenerated in the PLL circuit of the clock generator 26.

For recording, MFM may be used, but LP-MFM is preferable. Then, thelaser is pulsed once for each sub-mark and (at least) once for eachsub-space. A practical solution could be an integer value of thesub-space/sub-mark length ratio and pulsing the laser at a frequencycorresponding to the length of the sub-mark instead of the channel bitlength. In any case, the laser's duty cycle (pulse shorter than thesub-mark length) and timing (phase between magnetic field and laserpulse) should be optimized.

In case of LP-MFM data recording, the laser of the optical pick-up unit30 is modulated with a fixed frequency corresponding to half of theperiod of the data channel clock, and the data recording area or spot ofthe rotating magneto-optical recording medium 10 is locally heated withequal distances. Additionally, the data channel clock output by theclock generator 26 controls the modulator 24 and the sub-mark adjustingcircuit 22 to generate a data signal with a standard clock period. Therecording data are modulated and code-converted by the modulator 24 toobtain a binary run length information corresponding to the informationof the recording data. In the sub-mark adjusting circuit 20, a markregion of the recording information is converted to at least onesub-mark portion and at least one sub-space portion, while a spaceregion corresponding to a channel bit is maintained. Thus, a code runlength consisting of a plurality of mark regions directly following eachother is converted to a number of subsequent sub-mark and sub-spaceregions having respective predetermined lengths L1 and L2, respectively,selected to obtain a desired stray field characteristic a the readoutlayer of the disk 10. The pattern of sub-mark and sub-space portions ofthe code data output from the sub-mark adjusting circuit 22 is forwardedto the phase adjusting circuit 18, and after phase adjustment, forwardedto the driver 14 via the recording/playback switch 16.

The structure of the magneto-optical recording medium 10 may correspondto the structure described in the JP-A-2000-260079.

According to the preferred embodiment, the sub-mark adjusting circuit 22is arranged to increase the stray field for larger domain lengths byinserting regions with opposite magnetization direction. By individuallyand continuously choosing suitable values for the lengths L1 and L2 ofthe sub-mark portions and the sub-space portions, respectively, thestray field characteristic of short domains and large domains can beoptimized to ensure proper reading and improved power margin.

Furthermore, the stray field level at the readout layer can be furtherimproved by increasing the distance between the storage and the readoutlayer, however, at the expense of a reduction in stray field amplitude.Some reduction in stray field shouldn't be a problem for stable readout,so that this approach can be used to optimize disc structures.

Additionally, it should be considered that the surroundings, i.e.neighboring run-lengths, also influence the stray field of a domainstructure, due to the long range of the magnetic dipole fields of thedomains. Therefore, the write strategy for a run length could also beadapted at the sub-mark adjusting circuit 22 based on the previous andthe following data. The length over which this influence is importantdepends on the disc structure, but is around a few 100 nm. For thermalprofiles (much) narrower than this, the influenced region will besmaller because the magnetization becomes rapidly smaller away from thecenter of the thermal spot.

The proposed write strategy leads to an increased flexibility andallows, in principle, to eliminate differences in readout conditions forall combinations of short and long run lengths. The first thing toconsider is that it is not necessary that the stray field is constantover the run length. The stray field needs to be high only where MAMMOSexpansion should occur. More generally, the only requirement for MAMMOSreadout is that there should be overlap between the coercive field andthe stray field from the bit pattern where MAMMOS expansion shouldoccur, e.g. by application of a sufficiently large external magneticfield. In the known simple write strategy, this can be achieved by usinga periodic bit pattern of up/down magnetized regions where theup-regions are synchronized with the expansion direction of the externalmagnetic field.

However, there is no need to use a periodic bit pattern inside a runlength as long as the pattern gives sufficient overlap with thecoercivity profile. Therefore it is possible to make use of the factthat the copy window (and with it the overlap) increases when the localstray field is higher than the minimum stray field to give MAMMOSreadout (for a given laser power and external field). In this way a longrun length, for example an I5 carrier, can be written with e.g. 3separate ‘up’-domains with length and separation adjusted to thecoercivity profile in order to give overlap at the required times.Hence, the sum of the length L1 of a sub-mark portion and the length L2of a sub-space portion may continuously change and no longer needs tocorrespond to the channel bit length b.

FIG. 4 shows a corresponding schematic diagram for an I5 in an I1carrier, according to first example of the preferred embodiment for achannel bit length b=100 nm. As indicated in the upper portion, eachchannel bit is divided into four units, and the magnetization directionfor each unit is indicated by ‘+’ or ‘−’. The black squares represent acontinuously written I5. The other curves show three separated peaks forthe 15 run length, with a stray field larger than the I1 carrier levelindicated by the dashed line. The resulting overlap (vs. time) obtainedas the coercivity profile (solid line, asymmetric ‘parabolic’) scansover the bit or domain pattern is shown below the stray field curves forthe down triangles. The overlap is continuous over the I5 run length.Thus, when applying a modulated external field H_(ext), as indicated bythe next lower waveform in FIG. 4, the resulting signal indicated by thelowest waveform in FIG. 4 is correct. Comparing the other curves, it isclear that the marks neighboring the I5 run length also need to bemodified to keep their stray field at the I1 carrier level (dashedline).

FIG. 5 shows a similar schematic diagram for a channel bit length of 80nm and a simple write strategy of L2/L1=1, where L1=L2=40 nm. The dashedcurves represent the I1 carrier, the squares show the 15 run length forthe conventional simple write strategy. The circles represent theproposed write strategy according to a second example of the preferredembodiment, in which only two marks are used to give correct MAMMOSreadout for the 15 run length, under readout conditions that allowcorrect readout of I1 carriers as well. Also in the second example, theoverlap is continuous over the 15 run length. Thus, when applying amodulated external field H_(ext), the resulting signal is again correct.

FIG. 6A shows a similar graph, but for a ratio L2/L1=3, where L1=20 nmand L2=60 nm. Although the total stray field is reduced, the flexibilityto choose the best bit pattern for the 15 run length is much greaterthan for L2/L1=1. The circles represent the conventional simple writestrategy, also with L2/L1=3. The overlap for the triangles is againshown in the next lower waveform, resulting in correct readout.

FIG. 6B is identical to FIG. 6A, but the overlap is drawn for a lowercoercivity profile, e.g. higher laser power. Correct readout is stillpossible, demonstrating an improved power margin. This increasedflexibility for larger L2/L1 ratios is mainly due to the fact that for(sub-)mark lengths below 80-100 nm, the stray field strongly increaseswith increasing bit length (see FIG. 2A). Moreover, the stray fieldincreases if the surrounding non-mark length (separation between marks)increases, i.e. for higher L2/L1 ratios. Thus, starting from a sub-marklength of 20 nm (for b=80 nm and L2/L1=3 like in FIGS. 6A and 6B), thereare many combinations of larger mark lengths and different separationsto reach the desired stray fields.

One of the boundary conditions in the optimization process is theminimum length of a domain that can be used, which is determined by thelimited modulation frequency of the field coil and its driver forwriting, and the requirements for sufficient S/N and good thermalstability for readout. Thus, the increased flexibility of the proposedapproach is very advantageous.

The proposed write strategy allows to write a long run length withsub-mark and/or sub-space lengths selected independent of the channelbit length. Thereby, the sum of the lengths L1 and L2 may arbitrarily bechanged, such that a long run length can be written with few well-chosendomains with a stray field larger than the minimum field for MAMMOSreadout. In this way, differences in readout conditions for allcombinations of short and long run lengths can be eliminated, resultingin substantially improved power margins for random data.

It is noted, that the present invention is not restricted to the abovepreferred embodiment and can be applied to any magneto-optical recordingprocess to reduce stray field variations and increase the read-outresolution.

1. A method of recording information as a pattern of marks and spaces ona recording track of a magneto-optical recording medium, said methodcomprising the steps of: a) writing a mark region by having at least onesub-mark portion of a predetermined first length magnetized in a firstdirection substantially perpendicular to a recording surface of saidrecording medium and by having at least one adjacent sub-space portionof a predetermined second length magnetized in a second directionopposite to said first direction; and b) changing the sum of saidpredetermined first and second lengths in dependence on said pattern ofmarks and spaces.
 2. A method according to claim 1, wherein saidchanging step is performed for said mark region based on patterns ofprevious and/or following marks and spaces.
 3. A method according toclaim 2, wherein the length of said patterns of previous and/orfollowing marks and spaces is a few hundred nanometers.
 4. A methodaccording to claim 1, wherein said sum of said predetermined first andsecond lengths is set to be greater than a channel bit length.
 5. Amethod according to claim 4, wherein the number of said sub-markportions in said mark region is smaller than the number of channel bitswhich correspond to the run length of said mark region.
 6. A methodaccording to claim 5, wherein a mark region with a run lengthcorresponding to five channel bits is written with two or three sub-markportions separated by corresponding sub-space portions.
 7. A methodaccording to claim 1, wherein said magneto-optical recording medium is adomain expansion recording medium comprising a storage layer and areadout layer.
 8. A method according to claim 7, further comprising thestep of setting the distance between said storage and readout layersbased on a difference between the largest and the lowest values of astray field along said mark region.
 9. A recording apparatus forrecording an information as a pattern of marks and spaces on a recordingtrack of a magneto-optical recording medium, said apparatus comprising:a) writing means for writing a mark by having at least one sub-markportion of a first predetermined length of said magneto-opticalrecording medium magnetized in a first direction substantiallyperpendicular to the recording surface of said recording medium and byhaving at least one adjacent sub-space portion of a second predeterminedlength magnetized in a second direction opposite to said firstdirection; and b) control means for changing the sum of saidpredetermined first and second lengths in dependence on said pattern ofmarks and spaces.
 10. A recording apparatus according to claim 9,wherein said control means is arranged to change said sum of saidpredetermined first and second lengths in dependence on the patterns ofprevious and/or following marks and spaces.
 11. A recording apparatusaccording to claim 9, wherein said control means is arranged to set thenumber of said sub-mark portions in said mark region to a value smallerthan the number of channel bits which correspond to the run length ofsaid mark region.
 12. An apparatus according to claim 9, wherein saidrecording apparatus is a disk player for a magneto-optical disk to beread by a domain expansion technique.
 13. A magneto-optical recordingmedium on which an information is recorded on a recording track as apattern of marks and spaces, wherein a mark region comprises at leastone sub-mark portion of a first predetermined length magnetized in afirst direction substantially perpendicular to the recording surface ofsaid recording medium and at least one adjacent sub-space portion of asecond predetermined length magnetized in a second direction opposite tosaid first direction, and wherein the sum of said predetermined firstand second lengths is changed along said recording track.
 14. Arecording medium according to claim 13, wherein said magneto-opticalrecording medium is a magneto-optical disk to be read by a domainexpansion technique.